I tried to test the nunit plugin by simply running the unit tests that are included in the NUnit v2.5.2 release. The TestResult.xml file is created appropriately in the workspace folder. Unfortunately the nunit plugin isn't able to publish the test results.

Command invoked as a build step:
================================
"C:\Program Files\NUnit 2.5.2\bin\net-2.0\nunit-console.exe" "C:\Program Files\NUnit 2.5.2\bin\net-2.0\NUnitTests.nunit"

Hudson log:
===========
[...]
Recording NUnit tests results
ERROR: Publisher hudson.plugins.nunit.NUnitPublisher aborted due to exception
java.io.FileNotFoundException: C:\Documents and Settings\user\.hudson\jobs\Run Unit Tests\workspace\temporary-junit-reports\TEST-NUnit.Core.Tests.Generic.DeduceTypeArgsFromArgs`2[[System.Double, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Int32, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TestMyArgTypes.xml (The filename, directory name, or volume label syntax is incorrect)
at java.io.FileOutputStream.open(Native Method)
at java.io.FileOutputStream.<init>(Unknown Source)
at java.io.FileOutputStream.<init>(Unknown Source)
at hudson.plugins.nunit.NUnitReportTransformer.splitJUnitFile(NUnitReportTransformer.java:108)
at hudson.plugins.nunit.NUnitReportTransformer.transform(NUnitReportTransformer.java:71)
at hudson.plugins.nunit.NUnitArchiver.invoke(NUnitArchiver.java:55)
at hudson.plugins.nunit.NUnitArchiver.invoke(NUnitArchiver.java:26)
at hudson.FilePath.act(FilePath.java:753)
at hudson.FilePath.act(FilePath.java:735)
at hudson.plugins.nunit.NUnitPublisher.perform(NUnitPublisher.java:102)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:601)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:580)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:558)
at hudson.model.Build$RunnerImpl.post2(Build.java:158)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:528)
at hudson.model.Run.run(Run.java:1280)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:129)